Neither word is more offensive than the other, or even more offensive than, say, 'dandelion'. Words cannot be offensive unless one person intends it to spite another, and the second person takes offense to its use. Not saying it doesn't happen (Lord knows it happens innumerable times every day on the internet), but a word by itself is not offensive without hate being put behind it.
'******' generally is considered a racial epithet towards black people, but many black people have taken the word and turned it into a term of endearment towards each other, substituting the 'er' for an 'a' and literally using it in the same context that one would use the word 'friend'. How many times have we heard people say about Person A "Yeah, that's my nigga!" when describing a close friendship? Personally, I heard it on a daily basis in high school, maybe not in so many words, but in the same context. They stopped letting the word offend them, and they turned it on its head.
'****' isn't, or wasn't' even an insult. It was just a slang term for female genitalia. People have started using it as an insult in the recent past, but that's not how it originally was used.
From the progression of **** one could argue that literally any word in existence could be repurposed into an insult of some sort.